Hedging is a summer pruning method involving the removal the distal or apical ends of shoots. In some parts of the world hedging is called topping or tipping. Like all pruning methods, hedging depresses vine growth vigor and capacity. It does so by reducing the number of leaves and buds, which contain potential leaves. Leaves, of course, function similar solar panels, converting solar energy into the chemical energy that drives vine growth and development. Because of its effects, hedging may either be a useful vineyard management tool or an impediment to fruit production and quality depending on when and how it is used. We will examine such uses in this article.

When is hedging useful?

It is useful in young vineyards during cordon establishment for enhancing uniformity among spur positions.

In the first step in cordon training a shoot originating a near the top of a trunk is tied to the cordon wire of the trellis. Once the shoot is horizontal, the shoot tip looses its dominance over the lateral buds, which are located at each node of the shoot in the axil between petioles and main stem. No longer held dormant by hormones produced in the shoot tip, lateral shoots emerge from the lateral buds to form future spur positions. In this discussion, these lateral shoots, which emerged from buds on the primary stem, are called primary lateral shoots.

The primary lateral shoots to emerge first are on the portion of the new cordon nearest the trunk. Having a head start, they quickly establish dominance, suppressing the emergence and growth of primary lateral shoots further from the trunk. Left unimpeded, the early emerging lateral shoots will form stronger spur positions at the expense of those at at the distal ends of cordon.

The dominance of early developing primary lateral shoots can be minimized by light hedging or tipping when they are eight to twelve inches long, removing only the shoot tip and the upper one to three nodes (fig 1). It normally takes two to three weeks for secondary lateral shoots to emerge and grow from the buds below the hedging cut on the primary lateral shoots. During this time, primary lateral shoots on the distal portion of the new cordon will continue to elongate, frequently reaching lengths similar to the hedged lateral shoots. (This assumes appropriate irrigation and fertigation.) By equalizing primary lateral shoot growth along the length of new cordons with hedging we have enhanced uniformity among future spurs positions.

It is useful for mitigating shoot length variability in producing vineyards.

Variability in shoot length is common in established vineyards. It occurs to some extent among shoots on individual vines, but marked differences in shoot length among vines within vineyards it is normally a greater concern. Variations in root zone characteristics, especially water holding capacity and fertility, are typically the basis for such variability. Irregular fruit production and ripening are immediate concerns, while overcropping leading to vine decline are common long-term concerns associated with restricted vine growth within highly variable vineyards.

Hedging is key part of a method for promoting shoot length uniformity. To this end, hedge canopies after fruit set and canopy development on normal growth vines is excessive (> 20 leaves or nodes per shoot). During such hedging, shorten only excessively long shoots to a length of around 18 to 20 nodes (fig 2, fig 3, fig. 4). To be successful, such hedging requires concurrent actions to improve root zone conditions in areas of lagging growth, such as modifying irrigation schedules for a continuous supply of moderate soil moisture and applying appropriate soil amendments and fertilizers. Early cluster thinning to avoid crop stress and redirect internal vine resources toward shoot elongation may also be beneficial for equalizing shoot growth.

When is hedging detrimental?

When it is used repeatedly and in place of other practices to control foliage growth and limit canopy size.

It is established fact – excessively large canopies and associated shading are detrimental fruit production, grape quality, and pest and disease management. Usually, excessive vegetative growth is a symptom of inappropriate vineyard management or vineyard design. Immoderate irrigations or nitrogen fertilization are common causes of too much foliage. Severe dormant season pruning resulting in too few buds per vine is another promoter of extreme shoot growth. Some vineyard designs restrict the number of buds per vine, inducing the same effect. High-density plantings with closely spaced vines on fertile soils are classic examples. In these cases, hedging is viticulturally inefficient, causing the unprofitable loss of invested water, mineral nutrients, carbohydrates, and time.

The detrimental effects of hedging become acute when hedging is applied too early or too severely to producing vineyards. Hedging any time before, during, or immediately following bloom will lower yields due to decreased fruit set. Also, early hedging while soil moisture is abundant will promote lateral shoot growth leading to increased canopy density, increased bunch rot, decreased fruit quality, and possibly, second crop. Severe hedging will delay ripening by reducing leaf area (fig 5). Many times severe hedging also exposes unacclimated fruit to direct sunlight, increasing sunburn and berry shrivel.

Conclusions

Hedging, like all vineyard practices, has potential risks and benefits. Both are controllable with properly set hedgers used at appropriate times. Under most circumstances, hedging established vineyards late, light, and in conjunction with regulated deficit irrigation and balanced mineral nutrient management is best.

If shoot thinning were a book or movie it might be entitled Pruning: The Final Chapter. Quite simply, shoot thinning, which is the selective removal of shoots, enhances the viticultural effects of pruning and in some ways, finishes the pruning job. Here we will explore the benefits of careful shoot thinning and how to achieve them.

Shoot thinning conserves and often, improves the shoot spacing established while spur pruning cordons (fig 1) or arms on head trained vines. It similarly sets the shoot density along canes retained as fruit bearing units (fig 2). In this way, shoot thinning lays the foundation for the fruit zone microclimate during ripening, especially with regard to air and light movement. It also aids the penetration of foliar sprays.

Shoot thinning refines the ratio of fruit to leaves that was roughly set during pruning. Usually, all sterile (unfruitful) shoots are removed during thinning. This lessens the shoot density with a minimum impact on crop level. Sometimes, however, it may be desirable to also remove shoots bearing clusters if they are too crowded or too numerous for timely ripening. As such, shoot thinning is a practice for achieving growth balance.

Careful annual shoot thinning can promote uniformity among spur positions. By removing higher shoots on spurs and arms while retaining lower shoots, shoot thinning keeps spur positions close to cordons, increasing their uniformity with regard to shoot growth vigor and fruit development. Where arms have crept far away from cordons, retaining a shoot (water sprout) arising from a latent bud on old wood low on an arm sets the stage for arm shorting and spur repositioning next winter (figs 3). Using these techniques, shoot thinning maintains vine form, facilitates other vineyard operations, and enables consistency in fruit production and quality.

Shoot thinning can also enhance uniformity among shoots within the growing season in which it is applied. To do so, remove the shortest and longest shoots, and retain those of average length. The effect of thinning to an average shoot length is improved synchronization of fruit development and correspondingly, enhanced grape quality.

Shoot thinning compliments pruning in one other very important way. Removing shoots during the spring leaves fewer shoots to be pruned the following winter. This has two outcomes. First, operational costs are reduced because properly timed shoot thinning is a faster operation than dormant pruning. Second, the risk of canker disease is reduced because, with fewer pruning cuts during the winter, there are fewer wounds exposed to infection.

Importantly, shoot thinning also reduces water and mineral nutrient requirements and accordingly, their costs. Further, it makes other, subsequent hand operations easier, like cluster thinning and hand harvest, making them less expensive. Typically, by diminishing canopy densities, shoot thinning eliminates the need for basal leaf removal. The exceptions may be varieties with large leaves, such as Chardonnay and Malbec.

Like all cultural practices the most critical questions for shoot thinning are “when?” and “how much?” (i.e. timing and severity). Early shoot thinning (shoot lengths ≤ 6 to 10 inches) is easier and less costly than later shoot thinning, but later shoot thinning often lessens the risk of leaf and crop loss from springtime environmental hazards such as wind and frost. Sometimes the decision to thin depends on forecasted weather and other site factors compared to the current shoot length, rate of elongation, and susceptibility of the variety to damage. Other times it is simply a matter of labor availability and other operational logistics.

A shoot density of about 5 to 6 shoots per foot of cordon normally achieves the benefits stated above. This frequently corresponds to removal of all shoots except those originating from count buds on spurs. For cane pruned vines, retain 1 shoot per cane node and 1 or 2 well placed renewal shoots per cane on the head of the vine. Less severe thinning reduces viticultural benefits. More severe thinning provides negligible additional benefit with a cost of lowered fruit yields (fig. 4).

In conclusion, shoot thinning is a viticultural practice that compliments pruning and like pruning, shoot thinning is simultaneously both a canopy and crop management practice. The objectives of shoot thinning are to attain a favorable ratio or balance between fruit and foliage, to promote light and air penetration into the fruit zone, to encourage uniformity in shoot and crop development along the length of a cordon or cane, to avoid canker disease, and to facilitate vineyard operations and control costs.

The rarity of early season frosts is among the many natural assets of California that lend themselves to wine grape growing. Still, when frosts occur the results can be economically devastating, with low revenues for the growing season due to significant yield loss (fig 1). In some instances, with prolonged, very cold temperatures, shoot damage and reduced yields are unavoidable. Fortunately, temperatures during many frost events are slightly below freezing and short in duration. In these circumstances, we can take actions in the vineyard to minimize or avoid frost damage. Our challenges are integrating frost avoidance strategies into our broader vineyard management plans and containing their costs.

Assessing frost risk is the first step in frost protection. Grapevines are most susceptible to frost damage when they are located at high elevations, in low-lying areas were cold air settles, and next to obstacles to air movement, such as orchards, tall riparian vegetation, levees, and buildings (fig 2). Even within vineyards that appear open and flat, there may be some sections that repeatedly sustain frost damage due to reoccurring settling of cold air. Obviously, vineyard managers must be particularly diligent to minimize frost damage in areas prone to freezing.

The second step in frost protection involves vineyard design. In frost prone areas, orient vine rows in the direction of predominant slope to promote air movement downslope and through the vineyard. In flatter areas, orient rows in the direction of the prevailing wind to take advantage of any air movement. Also, train cordons as high as feasible above the vineyard floor and the coldest air (fig 3). If possible, plant a variety that breaks bud late and has wooly (tomentose) foliar tissues because they less are likely to become frost damaged (e.g. Mourvedre, Barbera).

In established vineyards, prune to several shorter (1-bud) spurs rather than fewer longer spurs to spread risk & minimize the impacts of frosts. For added frost avoidance, mechanically pre-prune and delay final pruning as late as feasible (fig 4). Late pruning delays shoot emergence, hopefully to a time beyond the threat of frost. Shoots emerging from late pruned spurs may grow more rapidly and pass more quickly through the stage of greatest risk of crop loss because daytime temperatures are likely to be warmer. The period of greatest risk occurs while the clusters are near the shoot tip (E-L stage 12). This coincides with a shoot length of about 3 to 5 inches.

In frost prone areas, cultivate and pack the tractor rows, and maintain clean berms (fig. 5). Bare, packed soil absorbs heat during the day and radiates it at night more readily than soil covered with vegetation, dead or alive. Heat exchange is even greater when the soil is moist. In no-till vineyards, it is critical to mow cover crops short (≈ 4 inches) ahead of frost events to maximize heat transfer. Copper sprays, which kill certain bacteria (e.g. Pseudomonas syringe) that serve as condensation nuclei on foliar tissues, may lessen the incidence of frost damage.

To this point, our discussion of frost protection has involved passive measures before a frost. Where these alone are insufficient, we must adopt active measures that further modify a vineyards environment. Irrigation, especially overhead sprinkler irrigation, before and during freezing temperatures intensifies heat transfer between the soil and air, particularly bare soil. Artificial winds created by large fans circulate and mix air, and in so doing, prevent cold air from settling near the ground and vines. Fans are effective only if the air near the surface is colder than the air aloft (i.e during temperature inversions).

For vineyards damaged by frosts, there are a couple of management options. The first is to do nothing. This is the best option of vineyards with very little cluster damage and in most cases, the best option for cane pruned vines. Disadvantages of this strategy are associated with a flush of lateral shoots on the highest, undamaged nodes of the primary shoots. In many cases, these undamaged nodes are immediately above or within the fruit zone, which frequently leads to excessive shading and increased canopy management costs. The lateral shoots may also bear second crop, which can complicate fruit sampling and harvest scheduling.

The second option is removal of all shoots from all vines within a vineyard or a section vineyard that can be independently managed. In some instances, this can be the best option for spur pruned vineyards with extensive shoot and cluster damage. (Note: most flowers that are lightly frosted and appear yellowish or slightly brownish will cease development and not bloom). To be cost effective, the increase in yield from shoot removal must exceed the cost. Yield responses to shoot removal depend on vineyard location, variety, and pruning method. After the frost of spring of 2008, this method was successful in one out of four cases. In addition to shoot removal costs, this management option has the potential downside of delayed fruit maturation, although delays were inconsequential in trials in New Zealand. (In New Zealand, secondary buds of Chardonnay & Sauvignon blanc were “quite unfruitful”. Based on limited experience in California, secondary buds of Merlot are more fruitful than secondary buds of Cabernet Sauvignon, Tinto Coa, and Touriga Nacional.)

In the end, manage what you know is going to happen and the risks you have some control over. For everything else, purchase insurance to mitigate losses. In other words, do not sacrifice sound viticulture for fear of frost; rather incorporate frost protection measures, as appropriate, into your vineyard operation to optimize fruit production, grape quality, and sustainability.

It is a fact: chemistry is fundamental to life. It is also a primary means to directly influence grapevines, especially their mineral nutrition. Thankfully, our modern mineral nutrient toolbox includes a wide array of chemical tools for this purpose. To use them appropriately and effectively, we need to know about their characteristics and their behaviors in vineyards. In this article, we will focus on attributes and functions of common forms of applied nitrogen (N).

Before beginning, we need a bit of background information about the nature of N in vineyards. Vines acquire almost all N from soils in one of two mineral forms. They are the ions nitrate (NO3–) and ammonium (NH4+).

Fig. 1. Vineyard Nitrogen Transformations

There are no native sources of mineral-N in soils. Rather, all residual soil N occurs in organic forms contained in organic matter (OM). Mineral-N availability in soils depends on microbes to first decompose OM and release the organic-N it contains and afterwards to transform organic-N into mineral-N (fig. 1). The supply of mineral-N in soils is transient and normally low because most California vineyards soils are low in organic matter and because vines, cover crops, and soil microorganisms rapidly consume available mineral-N.

We will now consider nitrate. To maintain charge neutrality, negatively charged nitrate ions in fertilizers are bound to positively charged ions (cations). Nitrate fertilizers include calcium nitrate, mined sodium nitrate, and potassium nitrate. Of these, calcium nitrate is the most commonly used, with sodium nitrate limited to organic managed vineyards and potassium nitrate mostly used for foliar applications (table 1).

All nitrate fertilizers are highly soluble and quickly become available for uptake in soil solutions. Soil nitrate is highly mobile and readily flows towards roots as they take up water. Nitrate moves into roots both passively with soil water and actively across cell membranes. After it is inside roots, nitrate easily moves upwards to shoots where it acts fast, darkening foliage, stimulating photosynthesis, and prior to ripening, promoting growth.

For charge balance, positively charged ammonium ions in fertilizers are bound to negatively charged ions (anions), such as sulfate, thiosulfate, phosphate, or polyphosphate. Ammonium polyphosphate (10-34-0) is the base for most liquid N-P-K fertilizer blends, such as 3-12-14.

Ammonium fertilizers readily dissolve in water like nitrate fertilizers, but the N they contain is more slowly available for the following reasons. While vine roots directly take up some ammonium, certain bacteria convert most to nitrate before roots absorb it. The ammonium to nitrate conversion usually takes 1 to 2 weeks. Ammonium may also interact with the soil matrix, either being adsorbed onto particle surfaces or fixed between layers of certain clay minerals (table 2).

Actually, ammonium is potentially toxic to grapevine tissues. To protect them, ammonium taken into roots is immediately incorporated into organic compounds (amino acids and amides). Unlike nitrate, after uptake most N from ammonium remains in the roots and benefits them.

Urea (CO(NH2)2) is a third common form of fertilizer N. It is a familiar dry N fertilizer (46-0-0) and part of some liquid fertilizers, including UAN-32. Roots can absorb urea, but most passes through microbe-mediated conversion processes to ammonium and nitrate before uptake. For this reason, urea is slower acting than both nitrate and ammonium, and correspondingly, it has a longer residence time in soils and elicits slower responses in grapevines.

Before the advent of manufactured fertilizers, organic amendments, such as manures and cover crop residues rich in legumes, were important sources of applied N for vineyards. Today, they remain viable options for substantially increasing soil N. However, their low and variable N content and their slow and variable rate of N release make their contributions to vineyard N difficult to predict (table 2). For this reason, they often work best when used in moderate amounts in combination with other N fertilizers. In this role, they supply modest amounts of soil nitrogen for sustaining normal early season vine growth while, at the same time, providing numerous other benefits to vineyards associated with organic matter additions.

Actually, a combination of applied N forms is better than continued use of a single form for several reasons. First, it facilitates balanced vine nutrition because nitrate enhances the uptake of nutrient cations, like potassium, magnesium, and calcium, while ammonium enhances the uptake of nutrient anions, including phosphorus and sulfur. Second, it provides the greatest growth and development benefits, with nitrate mainly benefiting shoots and ammonium mainly benefiting roots. Third, it promotes soil pH neutrality because the alkalizing effects of nitrate cancel the acidifying effects of ammonium and urea.

Like all farming inputs, N fertilizers have associated risks. Nitrate, being highly mobile, is easily leached below the root zone. It may also be lost to the atmosphere if soils remain saturated for prolonged periods. Ammonium may volatilize as ammonia from warm, wet soil surfaces, as will urea. Urea may also gas off decomposing organic amendments. All forms of N, including organic-N, may be lost with surface runoff and soil erosion. To avoid such losses, apply N at moderate rates timed to satisfy the needs of specific developmental stages, rapidly incorporate it, and avoid over irrigation. Also, slow surface flow and protect your vineyard topsoil from erosion with a cover crop.

To summarize, nitrate, ammonium, and urea are the principal forms of N in fertilizers. They act differently in soils and vines. At the same time, they are complimentary, promoting balanced vine nutrition and soil pH neutrality.

For over 130 years, rootstocks have been available to protect grapevines form soil borne pests. From the beginning, rootstocks have displayed varying influences on the composite grapevine created by grafting. These include influences on grapevine growth, fruit yield, and other viticulturally important vine attributes. During this time, researchers have chronicled many rootstock influences in field trials and controlled environments, and the published record of their findings is extensive (see selected works in the Further Reading section of this article).

And yet, in spite of the long history of rootstock use and research, selecting a rootstock remains a dubious business due to uncertainty about rootstock influences for any specific vineyard situation. At the same time, the implications of a rootstock selection for a vineyard enterprise are profound, as they include long-term impacts on productivity, efficiency, and profitability. In this article, we will systematically examine important considerations in the rootstock selection process.

Soil Borne Pest resistance

The primary purpose of rootstocks is protection from soil borne pests. So, logically, they are the first consideration in rootstock selection. Almost all commercially available rootstocks have sound resistance for Phylloxera, the common insect pest in vineyard soils, and the rootstocks that do not are well known (e.g. AxR#1).

Many soils also harbor plant parasitic nematodes, which are microscopic worms. There are several types of parasitic nematodes, including both free-living (ectoparasites) and those that spend part of their lives in vine roots (endoparasites). No rootstock can tolerate all types of nematodes and only a few can tolerate most (fig. 1). These include Freedom, RS-3, and RS-9. Root knot nematodes are the most common endoparasites and where populations are high, Dogridge and Salt Creek are additional rootstock options. The rootstocks 5BB, SO4, and Schwarzmann, due to their moderate resistance, are suitable for many soils supporting only free-living nematodes.

Over time, soil borne pests adapt to resistance mechanisms in rootstocks, which reside in their genes. For this reason, when replanting a vineyard site, it is best to choose a rootstock with a different genetic composition (tables 1 and 2). For instance, if you plan to replant a vineyard that is currently on 101-14 you would avoid 3309 or Schwarzmann as replacement rootstocks because they are hybrids with similar parentage.

Influence on Grapevine Growth Vigor and Capacity

Usually, the influence of rootstocks on grapevine growth vigor and mature vine size is the second consideration while selecting a rootstock (table 3). On deep, well drained and fertile soils, vines on most rootstocks are more vigorous and larger than ungrafted vines. And a few, like Dogridge and Salt Creek, are extremely invigorating. There are, however, a few rootstocks that are somewhat devigorating, including 420A, Schwarzmann, and 161-49C. On deep and fertile soils, these rootstocks are a good match for highly vigorous varieties, like Syrah, Sauvignon blanc, and Chenin blanc. They are also the most appropriate rootstocks for high-density plantings.

Scion x Rootstock Interactions

Among rootstock selection considerations, the scion variety grafted on top of a rootstock is just as important as the soil below a rootstock. In fact, the scion variety usually has a greater influence on the composite vine than the rootstock. Apparently, the carbohydrates and hormones produced in the top of vines are more important to overall vine function than the mineral nutrients taken up and the hormones synthesized in the bottom of vines. As a consequence, experiences with a specific rootstock may not be transferable from one variety to another.

For example, on a deep clay loam soil, Schwarzmann had a marked devigorating affect on Viognier, while having a negligible influence on the growth vigor of Sauvignon blanc. During this part of the selection process, seek the guidance of others experienced with the variety you plan to plant, including fellow grape growers and farm advisors.

One important interaction of scion and rootstock involves the Bordeaux varieties, including Cabernet Sauvignon. They have a tendency for erratic budbreak and shoot emergence, frequently leading to blank spaces in the centers of cordons. This tendency is exacerbated by most rootstocks with V. berlandieri x V. rupestris parentage, including 1103P and 140R.

Viruses

The virus status of the scion variety and clone is the final consideration in rootstock selection. When using non-CDFA certified clones or clones of uncertain virus status, avoid 3309, Freedom, and 5BB. Vines on these rootstocks commonly display more severe disease symptoms than vines on other rootstocks. Of course, using only CDFA certified plant materials, which greatly decreases the risk of harmful virus diseases, is always the most prudent option.

Conclusions

In summary, select rootstocks based on soils pests, other soil limitations, and the scion variety and clone (fig 2). Several less well known, but commercially available rootstocks were not mentioned in this article. Our lack of knowledge about them represents lost opportunities to further fine-tune our rootstock selection options for the best possible match with our scion variety, soil, and vineyard management objectives. After over 130 years, the need for rootstock research is still with us.

Cirami, RM. Interaction of Semillon clones with vine rootstocks (Does the clone of the scion affect the performance of the grafted vine?). The Australian Grapegrower and Winemaker. pp. 150-151. Annual Technical Issue. 1993.